READ-SUPPRESS.17 is impossible to satisfy
(let ((*read-suppress* t)) (read-from-string "#\GARBAGE")) is unwinnable. How can the reader know that the reader macro has terminated its reading? if it encounters "#\G1((((" it cannot infer that it should stop reading when it encounters parens; maybe the reader macro just gobbles up five characters whatever they are.
An unknown reader macro can consume an arbitrary number of characters from the reader string before returning; it can read a Malbolge program and execute it. So when a read-suppressed reader encounters an unknown sharpsign macro, I guess it loses in the general case - despite the standard specifying that in the examples page for
Also, where does it say in the spec that ";; Undefined macro dispatch characters should not signal an error"?
The best thing an implementation can do is to assume that a standard Lisp object follows the sharpsign macro I guess, but it is trivial to confuse the Lisp reader nonetheless since nothing mandates that a valid Lisp object can follow the reader macro:
(defun foo (stream dispchar char) (declare (ignore dispchar char)) (loop repeat 5 do (read-char stream) finally (return nil))) (set-dispatch-macro-character #\# #\G 'foo) (read-from-string "(#G1(((()") ;=> (NIL) (let ((*read-suppress* t)) (read-from-string "(list #G1(((()")) ;=> NIL (let ((*read-suppress* t)) (read-from-string "(list #H1(((()")) ;=> error
Therefore I'd argue that it is impossible to win with an unknown sharpsign macro despite the specification mandating this behaviour, and the only sensible option is to signal an error, which fully invalidates test case